Fringe is back, and after agent Olivia Dunham has escaped her kidnappers from the previous episode and is back to work, the science starts to get wacky again, this time regarding the common cold and some unusually large parasites.

Parasites were back on Fringe, after making a factually-challenged appearance in an earlier episode. In last night's episode, "Bound," Boston College professor Stewart Kinberg reaches the high point of a lecture about the microbial world, ruminating on how millions and millions of microorganisms like viruses and bacteria feed on us every day. Like many remarks in Fringe, the professor's words turn out to be prophetic--he suddenly begins to struggle to speak, then breaks into a fit of choking and falls to the ground. The teacher's assistant tries CPR to revive him, but in vain. And a moment later, the professor's killer makes its appearance.

What emerges from Kinberg's throat is a marvel of disgust--a slimy, spiny slug, about six inches in diameter, forces its way out of the professor's mouth and then scurries around the room, trying to escape. Agent Olivia Dunham learns that Kinberg had been about to move to a position as an epidemiologist for the Centers for Disease Control, and fearing that another CDC candidate could be in danger, she brings him into protective custody. But after taking a sip of water, he too convulses and dies, and a similar slug crawls out of him.

Walter's analysis of the slug-like parasite that he and Peter coral in the Boston College lecture hall tells him that the slug makes its way into people as tiny eggs, and stomach acid acts as the catalyst that causes the parasite to grow. Scott Gardner of the University of Nebraska-Lincoln's Manter Laboratory of Parasitology tells PM that this part of the case is fairly realistic. Parasites are endlessly creative and have plenty of ways to get inside you. "There's probably 100 different ones that get in," he says.

Some, like nematodes, often are ingested as eggs and then "hatch" in your gut. According to Gardner, the worm is already in a larval stage inside the egg--the host's caustic stomach acids wear down the egg and allow the nematode to break free and grow.

Could an overgrown common cold virus "cell" turn into a vicious killer?

Deeper examination of the nasty slugs leads the agents into truly junk science, however. Walter, in one of his brilliant, out-of-nowhere declarations, claims he has discovered that the spiny slug is really just one huge single cell, and not just any single cell--it's the virus that causes the common cold, blown up to the size of a football.Walter delights in the sweet irony of epidemiologists being killed by the common cold.

But this scenario is silly for several reasons. First of all, there is no "common cold virus," according to Carol Post, Purdue University professor of medical chemistry and molecular pharmacology. Scores of slightly different viruses, mostly rhinoviruses, can cause a cold.

Secondly, viruses are not cells. They're crafty bits of genetic code that must infect another organism's cells and borrow the host cell's machinery to reproduce. This peculiar way of life has led to years of debate over whether or not a virus qualifies as life. But, Post says, it most certainly is not a cell.

Lastly, while there are individual cells that can grow quite large, like the ostrich egg example that Walter gives, Post says that you can't just program a cell (or the flu virus) to supersize into a bigger version of itself. Basic physics prevents that from being possible for cells, and just about every other animal or organism. "No, that's science fiction," she says.